The present invention relates to a progressive wave type ultrasonic motor.
A typical ultrasonic motor has a stator and a rotor. The stator includes a piezoelectric element. The rotor is pressed against and rotatable relative to the stator. When an axial vibration having a resonant frequency is applied to the stator, the piezoelectric element vibrates axially, which generates axial vibration on the upper surface of the stator. The axial vibration on the stator is transmitted to the rotor, which generates a torsional vibration in the entire rotor. The torsional vibration rotates the rotor in a predetermined direction.
The amplitude of the torsional vibration in the rotor is preferably large. That is, the greater the amplitude, the greater the driving force of the rotor and thus the more efficiently the rotor rotates. To increase the amplitude of the torsional vibration, an ultrasonic motor having a rotor 91 shown in FIG. 14 has been proposed.
Rotor slits 92 are formed in the outer surface of the rotor 91. The rotor slits 92 promote the torsional vibration generated in the entire rotor 91. That is, the slits 92 increase the amplitude of the torsional vibration, which efficiently rotates the rotor 91.
The rotor 91 is rotated by generating torsional vibration in the entire rotor 91. In this case, the smaller the axial dimension of the rotor 91, the less the driving force of the rotor 91. That is, if the axial dimension of the rotor 91 is decreased, it is difficult to generate axial vibration in the entire rotor 91. Specifically, if the rotor 91 is excessively flattened, a bending vibration as in a plate is generated in the rotor 91, which hinders torsion of the rotor 91. Therefore, the axial dimension of the rotor 91 cannot be decreased beyond a certain limit. This limitation of reduction in the rotor axial direction limits the reduction of the rotor size, which prevents the size of the ultrasonic motor having the rotor 91 from being decreased.
Accordingly, it is an objective of the present invention to provide a compact ultrasonic motor.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an ultrasonic motor is provided. The motor includes a stator vibrated by a piezoelectric element and a rotor. The rotor includes a plurality of contact portions arranged about an axis of the rotor. The contact portions contact the stator. Each contact portion independently vibrates in response to the vibration of the stator, thereby causing the rotor to rotate.
The present invention may also be embodied as a method for designing an ultrasonic motor. The motor includes a stator, which is vibrated by a piezoelectric element, and a rotor, which rotates in response to the vibration of the stator. The method comprising: computing a resonant frequency of the stator, wherein an alternating current having the computed frequency or a frequency close to the computed frequency is applied to the piezoelectric element; forming a plurality of contact portions on the rotor that are arranged about the axis of the rotor and contact the stator; and designing each contact portion such that each contact portion has a resonant frequency that is equal to or close to the frequency of a vibration generated in the stator.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.